1. Field of the Invention
The present invention relates to a server device, a mobile terminal, and a positioning mode selecting method configured to select one positioning mode out of multiple positioning modes.
2. Description of the Related Art
A positioning method has heretofore been disclosed, for example, in Japanese Patent Application Laid-open Publication No. 2005-20216. In this example, the positioning method is configured to incorporate a global positioning system (GPS) receiver in a mobile terminal and to specify a position of the terminal by use of only signals from GPS satellites or both of the signals from the GPS satellites and a signal from a cellular base station. In an outdoor environment where there are no obstructing objects, the mobile terminal can receive sufficient signals from GPS satellites. Accordingly, the mobile terminal can receive the signals from four or more GPS satellites in many places.
In the positioning method using the GPS satellites, values of X, Y, and Z as three-dimensional coordinates and time of reception by the terminal are unknown. For this reason, it is essential to receive the signals from at least four GPS satellites. That is, it is possible to specify the position only by use of the GPS when the terminal can receive the signals from four or more GPS satellites.
In the meantime, the mobile terminal is also used in various places such as on a street canyon surrounded by large buildings or in an indoor place. In these environments, the mobile terminal is often unable to receive the signals from four or more of GPS satellites because the mobile terminal cannot catch weak signals from the GPS satellites. When the mobile terminal can only receive the signals from less than four GPS satellites, it becomes impossible to measure the position of the terminal only by use of the GPS satellites.
As a countermeasure for such a circumstance, a hybrid positioning mode is disclosed so as to achieve measurement of the position of the mobile terminal even when the terminal can receive the signals from less than four GPS satellites. The hybrid positioning mode is configured to treat a cellular base station as one of a signal emission source instead of the unreceivable GPS satellites.
Cellular base stations are treated as equivalent to the GPS satellites by measuring timing of signals transmitted from the cellular base stations so that four or more signal sources are obtained. In this way, it is possible to calculate the position of the mobile terminal in various places. In a special case, it is possible to calculate the position of the mobile terminal by receiving four or more signals that are transmitted from the base stations even when there are no receivable signals from the GPS satellites.
The above-described mode requires at least four receivable signal sources including the GPS satellites and the cellular base stations. The number of signal sources receivable by the mobile terminal varies depending on the surrounding condition of the mobile terminal, and in some places, the number of the receivable signals is less than four.
There is disclosed a cell/sector positioning mode as a countermeasure in this case. Specifically, the position of the cellular base station that covers a location of the mobile terminal is calculated as the position of the mobile terminal. When the cellular base station is sectorized itself, the center of each sector is applicable to the position. Meanwhile, it is possible to calculate a distance between the cellular base station and the mobile terminal when round trip time (RTT) is measurable. In this case, it is also possible to measure the position of the mobile terminal by use of this distance and sector information. The method of measuring the position of the mobile terminal by use of the cell/sector positioning mode is utilized as a fallback mode for a place where the GPS positioning or the hybrid positioning is inapplicable. This method can measure the position of the mobile terminal in anywhere of the service area for the mobile terminal.
As described above, there are several positioning modes for the mobile terminal. For example, there is a method which has been disclosed in: Text of a symposium “Applications and service development of a high-precision positioning system,” held on Mar. 3, 2006 pp. 43-45. In this method, the positioning modes are selected among the GPS positioning, the hybrid positioning, and the cell/sector positioning in accordance with the location measurement results of the mobile terminal.
According to this document, the positioning mode is changed based on the number of receivable signal sources. Specifically, either the GPS positioning or the hybrid positioning is selected when there are four or more signal sources, and the cell/sector positioning is selected when there are less than four signal sources.